The G20210A polymorphism has been shown to alter the efficiency of prothrombin mRNA processing. Here we show that the G20210A mutation also alters prothrombin mRNA stability. Three-fold more prothrombin protein and mRNA were produced in NIH-3T3 cells transfected with the prothrombin cDNAs containing the 20210A variant compared to cells expressing the 20210G variant. mRNA stability assays using chimeric globin transcripts harboring the G or A variant of the 97 nt prothrombin 3′ UTR indicated that the 20210G variant conferred greater instability to the globin reporter transcript than the A variant in transfected HepG2 cells. Both variants of the prothrombin 3′-UTR were shown to provide binding sites for a number of cellular proteins including HuR, an RNA binding protein associated with mRNA stability. Our results indicate that the G20210A is a bifunctional polymorphism, as it not only alters the efficiency of mRNA processing, but also the decay rate of prothrombin mRNA.